The present invention relates to a metal container having a trunk pipe and branch pipe, such as a sheath of gas insulated switch gear, and more particularly, to a structure of welded metal container having a trunk pipe and branch pipe welded thereto and flanges welded to the each end portion of the trunk pipe and branch pipe, and manufacturing method and apparatus therefor.
During manufacturing, a metal container having a trunk pipe and branch pipe, for example, a sheath of gas insulated switch gear which has a plurality of pipes welded to the trunk pipe, it is necessary to accommodate weld flanges to end portions of the branch pipe and trunk pipe. However arc welding is used for the above purpose. In the welding, however, this arc welding requires a complete penetration welding so as to ensure the gas-tightness. The flange faces have to be machined after welding when an accuracy is required. Laser welding is used for joining pipe and flanges, as disclosed in Japanese laid-open patent publication No. 59-189092/1984, Processing of LAMP '87, Osaka, May, 1987, pages 157-162, "Deep Penetration Welding with High Power Co Laser", HIRAMOTO et al, and Mitsubishi Denki Giho, No. 10, 1989, page 37-40.
The aforementioned prior art pays no attention to the working method, in which the joint between the flanges and trunk pipe is merely changed from the arc welding to the laser welding. No consideration is taken into the structure and manufacture of the actual metallic containers, namely the widely used metal container having a branch pipe such as the gas insulated switch gear sheath having branch pipes. Therefore, arc welding is still used for manufacturing the metallic container having a plurality of branch pipes.
When the trunk pipe and the flange are bonded by arc welding, the flange may fall down due to the welding heat and a restraint or support must be provided.
For example, with the following welding conditions:
diameter of the trunk pipe: 600 mm, PA1 length of the trunk pipe: 2,000 mm, PA1 thickness of the trunk pipe: 6 mm-12 mm, PA1 weld width (bead width): 6 mm-8 mm,
the welded flange is inclined or drops by about 2 mm. Therefore, inaccurateness in the degree of parallelization and perpendicularity occurs. In order to enhance the accuracy of the container, the flange have to be machined after welded, and the machining results creates problems.
Unless the ends of the branch pipe and the trunk pipe are precisely machined after welding, it is difficult to weld the trunk pipe to the flanges. This makes it necessary to machine the ends, but the flanges are inevitably deformed when welded. Therefore, it is necessary to maintain the accuracy of the flange by machining the same so as to set the flatness of the flange surface and distance between flanges to suitable levels and to eliminate the inclination of the flange surface after welding operation. This raises a serious problem during production.
The prior art requires a large-sized machine for machining the flanges for a large container. This necessity dilutes the effects of accomplishing the machine before the welding. However, this dilution is not taken into consideration to raise another serious problem in case of the large sized container.
In addition, in conventional techniques, a recess of 0.2-3 mm in depth is provided in the flange, and an end portion of the pipe is inserted in this recess. A laser beam is applied to the portion to be welded, in the direction to carry out the laser welding of the workpieces while rotating the same.
The angle at which the laser beam is applied to the surfaces to be welded is about 3.degree.-20.degree.. That is, a laser beam is applied diagonally to the surfaces to be welded. Therefore, the depth of penetration of the flange and pipe is uneven, and perfect penetration bead weld cannot be obtained in some cases in one pass.
In general, the outer diameter of a pipe has tolerance of .+-.1% according to the Japanese Industrial Standards. Therefore, even when a recess of a nominal pipe diameter is cut in one surface of a flange, it becomes difficult to insert a pipe, the outer diameter of which has certain tolerance, in the recess in the flange or a clearance between the pipe and recess becomes large even if the pipe fits in the recess, so that such flange and pipe become in some cases unsuitable for welding.
Accordingly, if a pipe of about 50 mm (about 2 inches) in diameter is welded to a flange by using the above-described conventional welding techniques, the pipe can be inserted in the recess in the flange due to the small tolerance of this diameter and the welding can be carried out. However, when a large-sized pipe having a diameter of, for example, around 744 mm is used, the welding thereof to a flange cannot be done suitably in many cases since this pipe has tolerance of as large as .+-.7.4 mm.
A laser machining apparatus for cutting or welding a workpiece by making use of a laser beam from a laser oscillator is disclosed in, for example, Japanese Laid-Open Patent Publication No. 60-227987/1985.
A machining table is sequentially arranged thereon with a welding head, a cutting head and a heat treatment head and is underlaid by a laser oscillator so that the workpiece can be moved in the same direction as a laser beam outputted from the laser oscillator. Moreover, the welding head and the cutting head on the machining table can be moved at a right angle with respect to the output direction of the laser beam.
In machining application of the workpiece, either the welding head or the welding head and the cutting head are moved at a right angle with respect to the output direction of the laser beam so that the laser beam may not be obstructed but can enter the machining head. The workpiece is so moved that the workpiece may be positioned just below the machining head in operation.
Since the individual machining heads are independently fixed on the machining table in accordance with the prior art, the machining heads have to be moved each time according to the machining application of the workpiece, and the workpiece must be positioned just below the machining head in accordance with the machining application, thus raising a problem in the machinability. Moreover, the individual machining heads may be spaced at distances for accommodating small or large workpieces. As a result, a problem arises in that the system is of a large size. When the machining head once moved is returned to its initial position before a predetermined machining, it is necessary to align the optical axis of the laser beam coming from the laser oscillator and the center of a bend mirror which is disposed in the machining head to change the path of the aforementioned laser beam. This alignment is difficult, and it is seriously troublesome to position the moved machining head for each machining. Moreover, the aforementioned prior art does not take into consideration the cutting of the end face of the tubular container such as the bus of the gas insulation control apparatus nor into the correction of that gap between the pipe end and the flange groove, which never fails to occur in the actual welding, and the correction of the dislocation.